1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system such as a laser printer, an electrophotographic copying machine and so on, particularly to an image forming apparatus employing a two-component developing system, and a method for revising an image density in the image forming apparatus.
2. Related Art Statement
In an image forming apparatus employing an electrophotographic system which employs a two-component developing system, a developer comprising a carrier in the form of magnetic powder and a toner in the form of colored resin powder is inserted into a developing device, the carrier and the toner are mixed and stirred by a stirrer disposed within the developing device, and the toner is charged by friction. Further, where the toner is consumed by development, a toner is replenished into the developing device from a toner hopper, the replenished toner is mixed with the carrier and stirred, and charging is done by friction, similarly to that mentioned above.
The consuming quantity of toner per drive time of the developing device varies according to the printing rate or the number of prints per print operation.
Where the printing rate is low, it is natural that even if the drive time of the developing device is the same, the quantity of toner consumed is small, and therefore, the consuming quantity of toner per drive time of the developing device is reduced.
On the other hand, the reason why the consuming quantity varies according to the print number per printing operation is as follows: Before the start and after the printing operation, there is time during which the developing device is driven without developing. So, suppose that 999 sheets are printed, in case of intermittently printing every three-sheet, the time during which the developing device is driven without developing is accumulated and longer than that of the case of printing continuously. That is, where the printing number per printing operation is small, the consuming quantity of toner per drive time of the developing device is reduced.
Incidentally, there appears a phenomenon that in the two-component developing system, where the developing device is driven continuously in the state that toner is not consumed, the charge quantity of toner becomes changed due to the excessive friction. Such a phenomenon as described results from the properties of toner, but in a toner material whose charge quantity increases where the developing device is driven continuously, there poses a problem in that where the printing rate is low or where the printing number per printing operation is small, the consuming quantity of toner per drive time of the developing device is small so that the charge quantity of toner increases more than a proper value, because of which the image density lowers.
Conversely, in a toner material whose charge quantity decreases where the developing device is driven continuously, there poses a problem in that where the printing rate is low or where the printing number per printing operation is small, the consuming quantity of toner per drive time of the developing device is small so that the charge quantity of toner decreases more than a proper value, because of which the image density rises.
In any case, where the printing rate and the printing number per printing operation, that is, the consuming quantity of toner per drive time of the developing device is not taken into consideration, there occurs an inconvenience that the image density lowers or rises, resulting in absence of stability in printing state.
In view of the foregoing, as a method for overcoming such a problem as noted above to revise the image density, there has been proposed heretofore a method for calculating the printing rate on the basis of a pixel counter to thereby obtain information relating to the toner consuming quantity. However, the pixel counter is a device for integrating emitting time of a semiconductor laser to thereby obtain information, and therefore, an area of an electrostatic latent image formed on a photosensitive drum can be grasped, but the actual toner quantity adhered to the electrostatic latent image cannot be grasped. Since the adhered toner quantity depends on the oner charge quantity in the developer, information relating to the toner consuming quantity obtained is often uneven, and even if the printing condition is changed on the basis of that information, after all the image density could not be maintained in the stabilized state.
Further, as the other method for revising image density, there has been proposed a method for irradiating light on a batch-like toner image formed on a photosensitive drum, disposing a sensor for measuring reflecting light thereof, calculating the toner adhered quantity from the measured value, thereby changing the printing conditions. However, its sensor is so expensive that the whole apparatus becomes high in cost. Further, since the batch-like toner image has to be formed on the photosensitive drum, it is necessary to stop normal printing operation every time the image is formed, so that more toner is consumed, and the photosensitive drum is driven excessively, resulting in shortening the service life of the photosensitive drum.